Thousands of commuters face 24-hour rail strikes during the festive period due to another pay and staff dispute.
More than 3,000 South West Trains (SWT) employees will be balloted next Wednesday on industrial action which could fall at Christmas and over the New Year.
The Rail Maritime and Transport Union (RMT) will declare the result on Monday, December 17, and union chiefs expect the majority to back strike action.
SWT carries nearly 400,000 passengers a day in and out of Waterloo station.
The RMT wants SWT guards to receive a pay rise of 7.6 per cent, equivalent to that received by train drivers.
Another ballot relates to disciplinary action against two RMT members, who, the union claims, have been made scapegoats for the strikes over the summer.
Philip Bialyk, union representative for SWT, said: The offer that is on the table is no where near whats being offered to drivers. SWT undertook to treat everybody the same. Clearly thats not happening.
The RMT must give seven days notice of any action, which means that the first stoppage could take place on Christmas Eve.
Tony Adams, chairman of the Kingston Area Travellers Association, said: Its only fair that the guards receive the same percentage rise as the drivers.
If working conditions are better then the service will improve likewise.
The RMT has already rejected two pay offers made to the unions.
An SWT spokesman said the two staff members, Greg Tucker and Mick Skiggs, were disciplined over related incidents.
He said: We are more than happy to go to an independent body to decide the merits of the cases, but the RMT just seems determined to call another ballot.
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